1. Field of the Invention
An aspect of the present invention relates to an image forming apparatus, and, more particularly, to an image forming apparatus, which includes a pressure roller that is brought into contact with a heating roller to form a fixing nip.
2. Description of the Related Art
An image forming apparatus, such as, a printer, a photo-copier, a facsimile machine and a multi-functional product, is an apparatus used for printing images on sheets of print media in response to input image signals. One type of image forming apparatuses is an electrophotographic image forming apparatus. This type of image forming apparatus includes a light scanning unit to scan an optical signal corresponding to a target image onto a photosensitive medium, a development unit to develop the electrostatic latent image into a visible image by supplying toner to the photosensitive medium on which the electrostatic latent image is formed, and a fixing unit to fix the visible image once the image is transferred to a print medium, such as, a sheet of paper.
FIG. 1 is a side sectional view and FIG. 2 is a front sectional view schematically showing a conventional fixing unit. As shown in FIGS. 1 and 2, the fixing unit 1 generally includes a heating roller 3 having a heat source 2 installed therein, and a pressure roller 4 to be brought into close contact with the heating roller 3 to form a fixing nip N at a contact portion therebetween. The heating roller 4 includes a shaft 5 formed from a metallic material such as aluminum or steel, and a resilient layer 6 surrounding the shaft 5. As shown in FIG. 2, the shaft 5 is provided at either end thereof with a spring 7 to force the heating roller 3 to be brought into the close contact with the pressure roller 4 by elastically biasing the pressure roller 4 towards the heating roller 3.
As the sheet having the toner image transferred thereon enters between the heating roller 3 and the pressure roller 4, both of which are rotated, the toner image is fixed onto the sheet by heat delivered from the heating roller 3 and pressure between the heating roller 3 and the pressure roller 4.
To prevent the sheet from being crumpled due to the heat and pressure during such a fixing process, the pressure roller 4 may be machined to have a reversed-crown shape. That is, the pressure roller 4 has relatively large diameters at opposite ends thereof and a relatively small diameter at a central region thereof. As a result of the pressure roller 4 having the reversed-crown shape, the sheet has a higher linear velocity where the sheet is brought into contact with the opposite ends of the pressure roller 4 than where the sheet is brought into contact with the central region of the pressure roller 4. Thus, the sheet undergoes outward tension when entering the fixing unit that prevents the sheet from being crumpled.
However, since a conventional pressure roller has a free surface at either side of the resilient layer 6 that extends in a longitudinal direction of the sheet, there is a problem in that the resilient layer 6 is deformed in an axial direction of the pressure roller 4 as a result of the compression of the spring 7 (see Region A in FIG. 2). If the resilient layer 6 is deformed, the diameters of the pressure roller 4 at both sides are reduced, thereby weakening the crumple prevention ability of the pressure roller. This problem may also be caused if the pressure roller 4 is not machined to have the reversed-crown shape such that the sheet has a more rapid linear velocity at the central region of the pressure roller 4. Furthermore, when the pressure roller 4 is decreased in diameter due to deformation of the resilient layer 6, the width of the fixing nip created between the heating roller 3 and the pressure roller 4 is changed to an unexpected shaped degree, thereby making maintenance of a stable fixing performance difficult.